I think you're right. The speed radar works by detecting how fast something is moving away or toward it. A vehicle travelling through a turn (i.e. moving laterally with respect to the radar) would show a lower speed to the radar its than actual speed.
It seems likely that the actual speed is recorded in a straightaway directly toward the camera's speed sensor, (max speed in the line of sight, min speed across the line of sight, Then, after the rider passes the sensor, the camera takes the picture from behind in order to get the license plate. I'd bet a paycheck the speed displayed in the photo is not the speed at the time of the photo, but rather, the max speed since target acquisition. Just a guess, though.